The present invention relates to elemental sulfur production by the Claus process.
The most commonly used process for recovering elemental sulfur from sulfur compounds is the modified Claus process. So pervasive is the use of this process, It is difficult to imagine a world in which this process were replaced entirely by some other. It is well known in the art that what may appear to be small stepwise changes to the unskilled will most often have a dramatic impact on the capital or operating costs of production of thousands of tons of sulfur produced worldwide from refining and other operations using the modified Claus process. In addition, even the most egregiously polluting countries who in the past have neglected atmospheric discharge of sulfur compounds are in the process of tightening controls on that discharge, making the impact of improvements in the process even more global and important.
In the modified Claus process, one third of the H2S in an acid gas feed is oxidized thermally with air, enriched air or oxygen to form a certain amount of SO2. The latter then reacts with the remaining H2S to form elemental sulfur according to the Claus reaction, the complete reaction sequence is expressed as: EQU H2S+3/2 02{character pullout}SO2+H20 (1) EQU 2H2S+SO2{character pullout}3/nSn+2H20 (2)
However, although practically very difficult in processing waste or acid gases from other process operations, the ratio of H2S:SO2 must be tightly controlled at 2:1 in order to achieve maximum sulfur recovery with the modified Claus process alone. The term "maximum" sulfur recovery is somewhat misleading since maximum sulfur recovery by the modified Claus process is only about 90-98% due to the thermodynamic equilibrium limitations of the Claus reaction as illustrated in reaction (2) above. As a result, the residual sulfur content from a conventional Claus unit is still relatively high and may not meet emission requirements of the processing facility's location. The maximum recovery of elemental sulfur is limited by incomplete conversion of H2S and SO2 to elemental sulfur due to the reduced thermodynamic chemical equilibrium for the Claus reaction under the economically advantageous conditions required for the unit operation, i.e., at atmospheric pressure and in the presence of the several other chemical entities. The 90-98% elemental sulfur recovery also depends heavily on the H2S concentration in the feed gas and the two or three Claus catalytic stages used, among other factors.
To address the problem of the less than desired elemental sulfur conversion and recovery by the modified Claus process, a number of different methods exist to work in combination with the basic modified Claus process to increase that conversion and recovery. The first practiced set of methods used to increase sulfur recovery in combination with the modified Claus process was to further treat and/or recycle the gas issuing from the last Claus catalytic stage, i.e., the tail gas.
One of the tail gas treating processes, which increases the overall sulfur recovery from a conventional Claus unit, is the BSR/Selectox process. In this process, the Claus tail gas is first heated to a desired reaction temperature required in the BSR hydrogenation/hydrolysis catalytic reactor. Typically, a reducing gas generator (RGG) is used to heat the tail gas and provide additional hydrogen by incomplete oxidation of a hydrocarbon feed in the RGG. Sulfur species in the tail gas, such as SO2, COS and CS2, are converted to H2S in a hydrogenation/hydrolysis reactor. After the hydrogenation step, the excess water in this gas is reduced by condensation. The gas is then processed in the Selectox reactor for sulfur recovery. The Selectox catalyst directly catalyzes the oxidation of H2S to SO2 in the presence of oxygen. The Claus reaction for the production of elemental sulfur from H2S and SO2 is also catalyzed by the Selectox catalyst.
Stoichiometric amount of oxygen is added to the Selectox reactor in order to achieve an H2S:SO2 ratio of 2:1 for the Claus reaction. Overall sulfur recovery for Claus units equipped with BSR/Selectox unit can be up to 99 percent.
If even higher sulfur recovery is desired, the Selectox reaction step is removed and the converted-species sulfur in the form of H2S issuing from the hydrogenation/hydrolysis reactor is absorbed in an absorber using an H2S-selective amine process, such as MDEA. The removed H2S is recycled to the Claus thermal stage for enhanced sulfur recovery. This process, known as BSR/MDEA or SCOT, can improve overall sulfur recovery to more than 99.9%. However, the additional capital cost for the BSR type tail gas processes can be more than 50% of a conventional Claus unit because of the additional hydrogenation, water removal and the tail gas cleanup steps.
Another method to increase overall sulfur recovery for a modified Claus process is to use a selective oxidation catalyst stage as the last stage in which elemental sulfur is formed from non-elemental sulfur components in the tail gas. In a catalytic Claus stage, thermodynamic equilibrium limits the conversion of H2S and SO2 to sulfur. However, selective or direct oxidation of H2S to elemental sulfur is essentially complete in a selective oxidation stage and is not so limited. Where SO2 in the feed gas to a selective oxidation stage is sufficiently eliminated so it does not pass unreacted through that stage, a final conversion stage with selective oxidation results in higher overall sulfur recovery compared to a process using only catalytic Claus stages. The selective oxidation reaction is expressed as: EQU H2S+1/2 02{character pullout}1/nSn+H2O (3)
One of the commercial processes that use the final selective oxidation stage is the SuperClaus process. The SuperClaus process uses an H2S-shifted Claus operation (higher H2S:SO2 ratio than 2:1) in combination with a selective oxidation step. The SuperClaus consists of a thermal stage followed by two or three Claus stages and one final selective oxidation stage. The thermal stage and the Claus catalytic stages are operated under an H2S-shifted Claus operation, either by reducing the combustion air to the main burner of the thermal stage or by-passing a portion of the Claus feed gas around the main burner, in order to minimize the residual SO2 in the gas leaving the last Claus stage since any residual SO2 entering the selective oxidation stage will not be converted to elemental sulfur. Under this mode of operation, the H2S in the gas leaving the last Claus reactor is controlled, instead of controlling the H2S:SO2 ratio to 2:1 in a conventional Claus unit, before it is fed to the last selective oxidation stage in which H2S is oxidized to elemental sulfur. The selective oxidation catalyst employed is substantially insensitive to the presence of water vapor in the process gas and ineffective in promoting the reverse Claus reaction. The overall sulfur recovery for the SuperClaus process can be more than 99%, depending on the feed gas composition and the number of catalytic stages. By eliminating the hydrogenation step and the water removal step, as required by the BSR/Selectox, BSR/MDEA or SCOT process, the additional capital cost for substituting a SuperClaus stage for a 3rd Claus stage is claimed to be 15-20% higher than the Claus unit. However, operating the thermal stage and the Claus stages at higher H2S:SO2 ratio reduces the overall sulfur recovery efficiency of the front-end section of the process which results in a shift of the sulfur recovery load to the final selective oxidation stage. Due to normal plant fluctuations in acid gas feed composition and process conditions, the last catalytic oxidation stage can be subjected to high H2S feed and subsequent temperature excursion, which may require reactor bypass or plant shutdown.
Another process combines BSR hydrogenation of the above described sulfur compounds and the selective oxidation of H2S to elemental sulfur, eliminating the H2S-shifted Claus operation as required in the SuperClaus process. This process, represented in actual operation as the BSR Hi-Activity, and the SuperClaus 99.5 processes, can obtain up to 99.5% overall sulfur recovery. These processes are less expensive than the other types of BSR tail gas cleanup processes, because no water removal step is required. Although the unskilled may view the variety of Claus-combined sulfur conversion processes as easily understood, in fact sulfur plant operation is a very complicated and challenging job. Acid gas feed to a sulfur plant usually includes wide variation in the volume and concentration of sulfur and other compounds, including a substantial amount of ammonia or amine in some plants. Theoretically, control of the thermal stage(s) using air, enriched air or oxygen for conversion of H2S to SO2 has permitted some processes to obtain extremely high recovery of sulfur whether for the 2:1 ratio for H2S to SO2 or for H2S-shifted operation. In actual operation, the several interactions of stream component analysis and measurement of flow, temperature, pressure and other process parameters with the compressors, valves, burners, aging or fouled catalyst beds and other process equipment has made error-free, continuous recovery of sulfur from acid gas an elusive goal. The present invention makes a further improvement in the pursuit of that goal by eliminating or reducing the importance of such careful control of the H2S/SO2 ratio at 2:1 for the conventional Claus plants or H2S-shifted operation for the SuperClaus plants.